Creation of a Novel X ray Generator James Hostetter Louisiana State University and SLAC National Accelerator Laboratory

Why X rays? adapted from: http://constellation.gsfc.nasa.gov/public/resources/brochure/may04/xray_spectroscopy.html

Why short pulses?

Capabilities and Advantages 10’s of KeV X rays 1 picosecond pulses 100 MHz pulses 108 photons/s at 8 KeV Compact Relatively cheap to build Complementary to LCLS Always available to PULSE researchers

Current, Future and Past X ray Sources Shorter pulses means studying faster processes More photons means bigger signal May be unnecessary May be harmful LCLS is by far the brightest and fastest pulse source, but runs at very low repetition rate.

X-ray Production Cathode creates e- Potential difference accelerates electrons towards anode Collision with anode causes emission of X-rays E = q * V

Better X-ray Production Laser allows for pulsed x rays RF cavity compresses electron bunches Shorter pulses Brighter pulses

Photocathode basics GaAs Must be cesiated Must be exposed to NF3 or O2 Must have an ultra-high vacuum (UHV) Photocurrent decays ~10% Quantum Efficiency (QE) QE=e-/photon QE = I*h*ν Gate Valve Loadlock Laser Cesiator Manipulator Heater Ion pump P*e

Photocurrent Decay I = I0*e-t/τ ≈ I0 (1-t/τ) Half-life=2.36 days

Photocurrent vs. Power

Conclusion Future Work Significant Progress in the design of the x-ray generator. Understanding the photocathode is proceeding well Decay on the order of days Linear dependence of QE on power Future Work Scaling to short laser pulse and higher average current (1mA) Finish Electron gun design and sample hand-off

Acknowledgements David Reis Shambhu Ghimire Takashi Maruyama SULI program